Ladar receiver with enhanced signal to noise ratio and method

ABSTRACT

A laser receiver comprising a sensor; a first amplifier operatively connected to the sensor comprising a first gate, a first source and a first drain; a first subcircuit operatively connected between the first drain and the first gate comprising a first resistor, a first inductor and a decoupling capacitor configured to allow the first amplifier bias to be established by the at least one first biasing resistor; the impedance of the first gate being sufficient such that only a small proportion of the current from the sensor passes into the first gate; an inductor connecting the first gate to the at least one biasing resistor with high impedance at the receiver operating frequency; a second amplifier comprising a second gate operatively connected to the first drain; and an output configured to be operatively connected to a processing unit and a display unit configured to displaying output and method thereof.

GOVERNMENT INTEREST

The embodiments herein may be manufactured, used, and/or licensed by orfor the United States Government without the payment of royaltiesthereon.

BACKGROUND OF THE INVENTION

The present invention relates to a LADAR transmitting and receivingapparatus, and, more particularly, to a compact LADAR receivingapparatus with enhanced signal-to-noise performance and bandwidth.

According to Wikipedia, Lidar (Light Detection And Ranging or LightImaging, Detection, And Ranging) measures distance by illuminating atarget with a laser light. Lidar may be used for a variety of purposesincluding high-resolution maps (including airborne laser swath mapping(ALSM)) and laser altimetry. Lidar is alternately referred to as laserscanning or 3D scanning; with terrestrial, airborne and mobileapplications. Laser Detection And Ranging (LADAR) is an optical remotesensing technology that measures properties of scattered light to findrange and/or other information of a distant target. LADAR may be used ina variety of contexts for elastic backscatter light detection andranging (LIDAR) systems. Although the acronym LADAR is usuallyassociated with the detection of hard targets and the acronym LIDAR isusually associated with the detection of aerosol targets, there has beenno real standard on their use and both acronyms may be usedinterchangeably to describe the same laser ranging system. Accordingly,as used herein, the terminology LIDAR means LADAR and vice versa.

LADAR systems typically operate in the ultraviolet, visible, or nearinfrared spectrums, which gives a compact LADAR the ability to image atarget at a high spatial resolution and allows LADAR systems to be mademore physically compact.

As reported in U.S. Pat. No. 8,081,301, in order for a LADAR systemtarget to reflect a transmitted electromagnetic wave, an object needs toproduce a conductive or dielectric discontinuity from its surroundings.At radar frequencies, a metallic object produces a conductivediscontinuity and a significant specular reflection. However,non-metallic objects, such as rain and rocks produce weaker dielectricreflections, and some materials may produce no detectable reflection atall, meaning some objects or features are effectively invisible at radarfrequencies. Lasers provide one solution to this problem regardingnon-metallic detection. The beam power densities and coherency of lasersare excellent. Moreover, the wavelengths are much smaller than can beachieved with radio systems, and range from about 10 μm to around 250nm. At such wavelengths, the waves are reflected very well from smallobjects such as molecules and atoms. This type of reflection is calleddiffuse “backscattering.” Both diffuse and specular reflection may beused for different LADAR applications.

Some prior art LADAR systems have transmitter and receiver functionsthat rely on a co-axial or mono-static optical system that comprises acomplex assembly of beam splitters, polarizers, and steering mirrorsthat is very difficult to align, prone to losing alignment, subject tonarcissus, and requiring excessive space for a compact LADAR system. Asreported in U.S. Pat. No. 8,081,301, compact LADAR systems havegenerally been flawed by one or more factors including, lowpixelization, insufficient range or range resolution, image artifacts,no daylight operation, large size, high power consumption, and highcost. Prior art systems may use a wide bandwidth photodetector/amplifier system with a small detector, and a low shuntcapacitance, leading to a low signal-to-noise ratio or smallfield-of-view.

SUMMARY

A preferred embodiment of the present invention, inter alia, determinesrange to a target by measuring the time-of-flight of a light pulse froma laser to the target and return. Optionally, a two-axis MEMS mirror maybe used to direct the light pulse to a point in a scene and establishthe angular direction to a pixel. The preferred embodiment receiverlooks over the entire region scanned by the laser and produces a voltageproportional to the amount of laser light reflected from the scene. Theoutput of the preferred embodiment receiver may be sampled by ananalog-to-digital convertor. The net result may be a data filecontaining a range and a horizontal and vertical angle that identifiesthe position of every image voxel in the scene and its amplitude. Thisdata may be displayed on a computer using standard and stereo techniquesto render a three-dimensional image of the scene.

A preferred embodiment of the present invention comprises a laserreceiver comprising:

at least one of a tapered fiber bundle or light concentrator operativeto receive light;

a sensor operatively connected to the tapered fiber bundle or lightconcentrator, the sensor comprising a photosensitive region andoutputting a photocurrent;

a first amplifier configured to amplify the photocurrent operativelyconnected to the sensor; the first amplifier comprising a first gate, afirst source and a first drain, the operative connection between thesensor and the gate being configured to minimize inductance; the firstgate being operatively biased by at least one first biasing resistor;

a first subcircuit operatively connected between the first drain and thefirst gate; the first subcircuit comprising, in series:

-   -   a first resistor, a first inductor and a decoupling capacitor;        the first resistor effecting the photocurrent gain of the        amplifier; the first inductor effecting the bandwidth; and the        decoupling capacitor being configured to allow the first        amplifier bias to be established by the at least one first        biasing resistor;

the impedance of the first gate being high enough such that only a smallproportion of the current from the sensor passes into the first gate andthrough the first amplifier while a significantly larger portion passesthrough the first subcircuit;

an inductor connecting the gate to the at least one biasing resistorwith high impedance at the receiver operating frequency;

a second amplifier comprising a second gate with a gate impedancegreater than 200 ohms operatively connected to the first drain; at leastone second biasing resistor connected to the second gate with highimpedance for the purpose of decreasing the load impedance on the firstamplifier and thus increasing the voltage gain of the first amplifier;and thus enabling an increase in the value of the first resistor thatin-turn improves the signal-to-noise of the amplifier; the first andsecond amplifiers having a combined bandwidth sufficient to pass amodulated photocurrent at a frequency up to 500 MHz; the secondamplifier comprising an output configured to be operatively connected toa processing unit and a display unit displaying output from theprocessing unit in the form of pixels, the display being of sufficientquality to display an image of a target at 1000 meters;

whereby the receiver is configured to receive light reflected from atarget illuminated by a laser configured to emit light at an intensitysafe for human eyes and wherein a target is discernible in a range from0.3 to 1000 meters.

Optionally, the received light may be of a predetermined frequency andmodulation format. The first subcircuit may be a negative feedbackcircuit. Optionally, the sensor may comprise a large area photosensitiveregion with a diameter ranging from 1 mm to 10 mm which outputs aphotocurrent. The capacitance of the photodetector may be between 10 pFand 400 pF and the first amplifier may be an enhancement modepsuedomorphic high electron mobility transistor. Optionally, theinductance between the sensor and the gate may be minimized by makingthe distance between the photodetector and first gate approximately lessthan 1 cm. The inductance between the sensor and the gate may be in therange of 0.1 nH to 10. nH. The first resistor may be in the range of 20to 2000 ohms and the first inductor may be in the range of 50 nH to 2000nH such that the impedance of the bypass circuit causes the signal tonoise ratio, amplifier gain, and bandwidth to be optimal for aparticular lasar application. Optionally, the receiver may be used tooperate a robot and the receiver may operate on battery power providingcurrent in the range of 5 mA to 2000 mA in order to minimize the powerconsumption on the robot. The receiver may be sized so as to fit in anarea of less than or equal to 3 cm by 3 cm by 0.5 cm. Optionally, thereceiver comprises a plurality of subreceivers and the output of each ofthe plurality of subreceivers is substantially redundant and each outputis connected to a combining circuit that is operatively connected to aprocessing unit.

A preferred method of the present invention comprises:

-   a sensor, a first amplifier operatively connected to the sensor; the    first amplifier comprising a first gate, a first source and a first    drain, the operative connection between the sensor and the first    gate being configured to minimize inductance; the first gate being    operatively biased by at least one first biasing resistor;-   a first subcircuit operatively connected between the first drain and    the first gate; the first subcircuit comprising, in series; a first    resistor, a first inductor and a decoupling capacitor; the first    resistor effecting the photocurrent gain of the amplifier; the first    inductor effecting the bandwidth; and the decoupling capacitor being    configured to allow the first amplifier bias to be established by    the at least one first biasing resistor;-   the impedance of the first gate being high enough such that only a    small proportion of the current from the sensor passes into the    first gate and through the first amplifier while a significantly    larger portion passes through the first subcircuit;-   an inductor connecting the gate to the at least one biasing resistor    with high impedance at the receiver operating frequency;-   a second amplifier comprising a second gate with a gate impedance    greater than 200 ohms operatively connected to the first drain; at    least one second biasing resistor connected to the second gate with    high impedance for the purpose of decreasing the load impedance on    the first amplifier and thus increasing the voltage gain of the    first amplifier; and thus enabling an increase in the value of the    first resistor that in-turn improves the signal-to-noise of the    amplifier; the first and second amplifiers having a combined    bandwidth sufficient to pass a modulated photocurrent at a frequency    up to 500 MHz; the second amplifier comprising an output configured    to be operatively connected to a processing unit and a display unit    displaying output from the processing unit in the form of pixels,    the display being of sufficient quality to display an image of a    target at 1000 meters; the method comprising receiving light    reflected from a target in a range from 0.3 to 1000 meters;    processing and displaying the output.

These and other embodiments will be described in further detail belowwith respect to the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a prior art MEMS ladar andmirror.

FIG. 2 is a schematic of a prior art MEMS-scanned ladar receiver.

FIG. 3 shows the processing steps relevant to a MEMS-scanned ladar.

FIG. 4 is a block diagram of an example of the computing environment forthe preferred embodiment MEMS-scanned ladar data capture system.

FIG. 5A is a schematic illustration (Part I) of a hardware configurationof a preferred embodiment of the invention herein.

FIG. 5B is a schematic illustration (Part II) of a hardwareconfiguration of a preferred embodiment of the invention herein.

FIG. 6 is an illustration depicting a light concentrator for use with apreferred embodiment of the present invention.

FIG. 7 is a block diagram illustration of an eye safety system which maybe used in association with an alternate preferred embodiment of thepresent invention.

FIG. 8 is an illustration showing the hosels associated with lightconcentrators which may be used with a preferred embodiment of thepresent invention.

FIG. 9 is an illustration showing alternate photodetector for use in apreferred embodiment of the present invention. Shown to the left is athree-inch diameter HgCdTe/Si wafer. As shown to the right, each blockof the detector 40A comprises a 5×5 arrays of 1×1 mm detectors connectedwith wire bonds.

FIG. 10 is an illustration showing a detector using mercury cadmiumtelluride (MCT) on silicon.

FIG. 11 is an illustration showing an application of the presentinvention relating to an unmanned aerial vehicle (UAV) drone.

FIG. 12 is an illustration showing the visibility believed to beobtainable using a preferred embodiment of the present invention.

FIG. 13A is an illustration showing an example of ladar imagery.

FIG. 13B is an illustration showing another example of ladar imagery

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Preferred Embodimentsand the accompanying drawings in which like numerals in differentfigures represent the same structures or elements. The representationsin each of the figures are diagrammatic and no attempt is made toindicate actual scales or precise ratios. Proportional relationships areshown as approximates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments that are illustrated in the accompanyingdrawings and detailed in the following description. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale. Descriptions of well-known components and processingtechniques are omitted so as to not unnecessarily obscure theembodiments of the invention. The examples used herein are intendedmerely to facilitate an understanding of ways in which the embodimentsof the invention may be practiced and to further enable those of skillin the art to practice the embodiments of the invention. Accordingly,the examples should not be construed as limiting the scope of theembodiments of the invention. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the dimensions of objects and regions may be exaggerated forclarity. Like numbers refer to like elements throughout. As used hereinthe term “and/or” includes any and all combinations of one or more ofthe associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the full scope of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various ranges, elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. Forexample, when referring first and second ranges, these terms are onlyused to distinguish one range from another range. Thus, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toother elements as illustrated in the Figures. It will be understood thatrelative terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower”, cantherefore, encompass both an orientation of “lower” and “upper,”depending of the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below. Furthermore, the term“outer” may be used to refer to a surface and/or layer that is farthestaway from a substrate.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom less than one percent to ten percent and corresponds to, but is notlimited to, component values, angles, et cetera. Such relativity betweenitems ranges between less than one percent to ten percent. As may beused herein, the term “substantially negligible” means there is littlerelative difference, the little difference ranging between less than onepercent to ten percent.

As may be used herein, the term “significantly” means of a size and/oreffect that is large or important enough to be noticed or have animportant effect.

As used herein the terminology “substantially all” means for the mostpart; essentially all.

This description and the accompanying drawings that illustrate inventiveaspects and embodiments should not be taken as limiting—the claimsdefine the protected invention. Various changes may be made withoutdeparting from the spirit and scope of this description and the claims.In some instances, well-known structures and techniques have not beenshown or described in detail in order not to obscure the invention.Additionally, the drawings are not to scale. Relative sizes ofcomponents are for illustrative purposes only and do not reflect theactual sizes that may occur in any actual embodiment of the invention.Like numbers in two or more figures represent the same or similarelements. Elements and their associated aspects that are described indetail with reference to one embodiment may, whenever practical, beincluded in other embodiments in which they are not specifically shownor described. For example, if an element is described in detail withreference to one embodiment and is not described with reference to asecond embodiment, the element may nevertheless be claimed as includedin the second embodiment.

Embodiments of the present invention are described herein with referenceto cross-section illustrations that are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the elements in the illustrations are to be expected.Thus, embodiments of the present invention should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes. Thus, the layers or regions illustratedin the figures are schematic in nature and their shapes are not intendedto illustrate the precise shape of a layer or region of a device and arenot intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The present invention is directed to a novel approach to the design of aphotocurrent receiver required in compact, low-power, and low-cost laserradar (ladar).

The receiver circuit design is an improvement in terms ofsignal-to-noise ratio and bandwidth over a design described in a priorpatent “Ladar Transmitting and Receiving System and Method,” U.S. Pat.No. 8,081,301.

The ability of a laser radar to detect targets at a specified range andfield-of-view (FOV) is directly related to the amount of light capturedby its receiver front-end optics and following photodiode. Inapplications where the FOV is large, then the photodiode size must bealso large. At the same time the receiver bandwidth comprised of thephotodiode and following amplifier must be sufficiently wide to pass ashort duration pulse that may have a bandwidth in the range of 200-300MHz. When a large area photodiode is connected directly to aconventional microwave amplifier with an input impedance of 50 ohms, thephotodiode capacitance is large enough that the circuit bandwidth (i.e.receiver) is well below the requirement to pass the detected lightpulse. The present invention is directed to a laser radar receivercomprised of an amplifier circuit and large area photodiode thatprovides improved bandwidth and signal-to-noise (SNR) performance overthe prior design described in U.S. Pat. No. 8,081,301

The problem of designing a low-cost laser radar around a wide FOVreceiver using a large area photodiode has not been pursued becausedesigners realized that standard methods of photocurrent amplificationcould not attain the bandwidth and SNR needed to pass a short pulse anddetect the pulse at useful ranges. The present invention largely solvesthat problem.

Breadboard models of the receiver using various detector sizes andamplifier circuit parameters have been built and extensively tested.This work verified that the receiver SNR is improved 4-5 fold over theSNR obtained with the receiver design described in U.S. Pat. No.8,081,301 for equal bandwidths. More SNR may be gained through morejudicious circuit board layout in the future. This SNR improvement isimportant because it extends the range of ladar two-fold.

The present invention is useable in compact, low-power, and low-costlaser radars that may find use in autonomous small ground robots andsome large ground vehicles. Other system applications includesurveillance from small UAVs at short ranges and smart munitions. Laserradars with the receiver invention could find extensive use inautonomous automobiles, architectural engineering and designmensuration, and machine control. In the future, the receiver shouldfind use in other variants of the MEMs-scanned ladar described in U.S.Pat. No. 8,081,301. These ladars may serve in military applications suchas small ground robots, large ground vehicles, and other small UAVs andcommercial applications including autonomous automobiles, architecturalmensuration and design, and machine control.

FIG. 1 illustrates a schematic diagram of LADAR transceiver wherein atrigger signal from field-programmable gate array (FPGA) 10 commandslaser 15 (e.g., an Erbium fiber laser) to emit a short pulses of light(e.g., 2-3 ns pulses, at 200 kHz). These pulses of light are collimatedusing a GRIN lens, for example, and then directed to the surface ofmirror 20 (e.g., may be embodied as a small microelectromechanicalsystem (MEMS) mirror). In addition, analog voltages from high voltageamplifier (HV Amp) 25 control the pointing direction of mirror 20. Asthe pulses of light are reflected from mirror 20, they are subsequentlyfed into telescope 30 to amplify the reflected angle. Lightbackscattered upon hitting a target is collected on the large face offiber bundle 35, which may be tapered. Tapering of the fiber bundle 35effectively increases the diameter of photo detector 40 and therebyincreases the signal-to-noise ratio. Other means to increase lightimpinging on the photo detector such as light concentrators or lensesmay also be used. Photocurrent from photo detector 40 is fed intoamplifier 45, which may be embodied as a monolithic 50 ohm microwaveamplifier. The outputs of additional fiber taper/detector/amplifierchains may be fed into a summer 50 with additional filtering andsplitting functions to improve the ladar signal-to-noise ratio. Theoutput of the summer 50 after the filtering is fed into power dividerthat splits the signal into low gain channel 50 a and high gain channel50 b. In radio frequency (RF) interface board 55, both low gain channel50 a and high gain channel 50 b are adjusted in amplitude and limited inamplitude to produce output channels 60 a and 60 b. In addition, inputsto RF interface board 55 are summed with a photocurrent from anundelayed sample of the original transmitted light signal, shown asT-zero 55 c in FIG. 1. T-zero 55 c pulse of the transmitted signal isoptionally used as a reference to determine target range. Outputchannels 60 a and 60 b are subsequently fed into analog-to-digitalconverter (ADC) 60, shown in FIG. 1 as a two channel 8-bit ADC, viainput channels 60 a and 60 b. ADC 60 optionally samples input channels60 a and 60 b at a 1.5 giga-samples-per-second 45 (GSPS) rate forexample. This sampled data is fed to field programmable gate array 10,which stores the sampling data as a function of time from ADC 60 inmemory 65.

Memory 65 is optionally a first-in first-out register (FIFO), and startsstoring analog-to-digital converter (ADC) 60 sampling data upontransmission of the laser 15 pulse. In addition to storing sampling datafrom ADC 60, field programmable gate array (FPGA) 10 determines therange to the pixel, and formats the data for acquisition by computer 70for display. FPGA 10 also controls the pointing direction of mirror 20(e.g., via digital-to-analog converter (DAC) 12) and directs the laser15 to emit a pulse.

To increase the receiver capture area for a given size of photo detector40, a tapered fiber bundle 35 may be used to magnify the apparent sizeof a photo detector (e.g., a 1 mm diameter photo detector cantheoretically increase its effective diameter to 3.1 mm at the front ofa tapered fiber bundle 35, when the tapered fiber bundle 35 has amagnification ratio equal to 3.1:1). The theoretical maximal effect ofmagnifying tapered fiber bundle 35 is often not reached if photodetector 40 is a commercially packaged photo detector since thepackaging of the commercial photo detectors typically cannot couple theoutput of tapered fiber bundle 35 directly against the detector surfaceof photo detector 40 to capture all of the light. In addition, thecapacitance of photo detector 40 may limit the output bandwidth ofamplifier 45 (e.g., a photo detector with a 1 mm diameter detectingsurface may limit bandwidth to about 85 MHz when fed directly into a 50ohm microwave amplifier). This issue is addressed via an R-L-C circuit75 (shown in FIG. 2) between photo detector 40 and amplifier 45 outputto extend the bandwidth with a tolerable level of pulse distortion andstretching.

FIG. 2 shows an amplifier scheme in the prior art that solved thebandwidth problem identified in the previous paragraph. As shown in FIG.2, photo detector 40 may include a PIN InGaAs photo detector, which ispreferable to an avalanche photodetector. The principal for extendingthe bandwidth is based on a feedback circuit comprised of an inductor94, capacitor 93 and resistor 96.

Recent advances in Erbium fiber lasers have allowed Erbium fiber lasersto be manufactured as physically smaller units, and at a lower cost,than what was previously available. The electrical and opticalparameters of these new Erbium fiber lasers are identical to theprevious fiber laser technology except that the peak power has beenreduced, e.g., the peak power of a low-cost Erbium fiber laser may beone-fourth of the peak power of a current fiber laser. FIG. 2 showsreceiver 78 that compensates for a loss in signal power when using alaser with reduced peak power, according to the embodiments describedherein. Similar to FIG. 1, FIG. 2 includes fiber bundle 35 to enhancethe optical capture area. FIG. 2, however, uses a plurality of chipdetectors 40 instead of a single photo detector. For example, since eachPIN chip detector 40 may be only 1 mm square, four PIN chip detectors 40may be coupled to the output of fiber bundle 35. FIG. 2 illustrates asingle PIN chip detector 40. The output of each PIN chip detector 40 isfed into a separate e microwave amplifier 85. The output of eachmicrowave amplifier 85 is fed into an n-way microwave combiner 90.Microwave combiner 90 yields target signal 90 a that has an improvedsignal-to-noise ratio compared to the individual output of any ofmicrowave amplifier 85.

Although FIG. 2 uses a four-way microwave combiner 90, to match thenumber of PIN chip detectors 40 and microwave amplifiers 85 combinations(only one of which is shown in FIG. 2), the embodiments described hereinare not limited to a four-way microwave combiner and those skilled inthe art could readily increase or decrease the number of inputs tomicrowave combiner 90 to match increases or decreases in the number ofPIN chip detectors 40 and microwave amplifiers 85 combinations used.

The circuit shown in FIG. 2 also includes feedback circuit 92 positionedas a feedback loop for the output of microwave amplifier 85 to raise theoverall bandwidth. In feedback circuit 92, capacitor 93 comprises alarge capacitor used to decouple DC signal 94 at amplifier 85 outputfrom the input. Inductor 95 cuts-off the feedback path at highfrequencies, where the phase shifts in amplifier 85 may cause conditionsfor oscillations and if chosen correctly increases the circuitbandwidth. In addition, resistor 96 is effectively the actual feedbackelement over the bandwidth of interest. To further achieve stable (e.g.,non-oscillating) performance, amplifier 85 may include a wide bandmonolithic microwave integrated circuit (MMIC) amplifier (e.g., toprovide gain up to 7 GHz).

FIG. 3 illustrates a flow diagram according to an embodiment herein. Instep 100, the method of FIG. 3 describes receiving pulses of lightpulses of light via a tapered fiber bundle (e.g., tapered fiber bundle35), although this is optional. Step 101 describes producing a pluralityof photocurrents (e.g., as produced from photo detector 40, 40A or 40B).Step 102 describes amplifying the plurality of photocurrents using aplurality of microwave amplifiers (e.g., microwave amplifiers 45). Next,at step 103, the method of FIG. 3 provides a plurality of feedbackcircuits (e.g., feedback circuit 92 or 92A). Step 104 describescombining the plurality of amplified photocurrents using a microwavecombiner (e.g., microwave combiner 90). In step 105, the method of FIG.4 describes processing and displaying the combined output (e.g., viacomputer 70 or other processor).

The techniques provided by the embodiments herein may be implemented onintegrated circuit chips (not shown) with supporting computer code.

The embodiments herein can comprise hardware and software elements. Theembodiments that are implemented in software include but are not limitedto, firmware, resident software, microcode, etc. In addition, thehardware elements described herein may be simulated in software. Forexample, computer models of analog hardware elements described herein(e.g., lasers, microwave amplifiers, resistors, capacitors, andinductors) may be used in conjunction with emulators for discretehardware elements described herein (e.g., FPGA emulators) to simulateoperational parameters for the software elements of the embodimentsdescribed herein. Furthermore, the embodiments herein can take the formof a computer program product accessible from a computer-usable orcomputer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system. For thepurposes of this description, a computer-usable or computer readablemedium can be any apparatus that can comprise, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device. The medium can be anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system (or apparatus or device) or a propagation medium.Examples of a computer readable mediuminclude a semiconductor or solidstate memory, magnetic tape (as represented by 113 in FIG. 4), thumbdrive, a removable computer diskette, a random access memory (RAM), aread-only memory (ROM), a rigid magnetic disk (as represented by 111 inFIG. 4), and an optical disk. Examples of optical disks include compactdisk-read only memory (CDROM), compact disk-read/write (CD- R/W) andDVD. A data processing system suitable for storing and/or executingprogram code will include at least one processor coupled directly orindirectly to memory elements through a system bus. The memory elementscan include local memory employed during actual execution of the programcode, bulk storage, and cache memories which provide temporary storageof at least some program code in order to reduce the number of timescode must be retrieved from bulk storage during execution.

Input/output (I/O) devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers.

Network adapters may also be coupled to the system to enable the dataprocessing system to become coupled to other data processing systems orremote printers or storage devices through intervening private or publicnetworks. Modems, cable modem and Ethernet cards are just a few of thecurrently available types of network adapters.

A representative hardware environment for practicing a preferredembodiment of the present invention is depicted schematically in FIG. 4.This schematic drawing illustrates a hardware configuration of aninformation handling/computer system similar to that described in the'301 Patent, that may be used in conjunction with the embodimentsherein. The system comprises at least one processor or centralprocessing unit (CPU) 110. The CPUs 110 are interconnected via systembus 112 to various devices such as a random access memory (RAM) 114,read-only memory (ROM) 116, and an input/output (I/O) adapter 118. TheI/O adapter 118 can connect to peripheral devices, such as disk units111 and tape drives 113, or other program storage devices that arereadable by the system. The system can read the inventive instructionson the program storage devices and follow these instructions to executethe methodology of the embodiments herein. The system further includes auser interface adapter 119 that connects a keyboard 115, mouse 117,speaker 124, microphone 122, and/or other user interface devices such asa touch screen device (not shown) to the bus 112 to gather user input.Additionally, a communication adapter 120 connects the bus 112 to a dataprocessing network 125, and a display adapter 121 connects the bus 112to a display device 123 which may be embodied as an output device suchas a monitor, printer, or transmitter, for example

Optionally, a field programmable gate array (FPGA) may be utilized torun a code that controls the basic functions of the LADAR and does somepre-processing of the image data before display. A trigger signal fromthe FPGA may be used to command alaser to emit a short pulse of light ata rate of 200 kHz into, for example, a single mode fiber. The laser sizeis 9×7×1.5 cm LWH), the pulse is 2-3 ns long with an energy of 2 μJ, andthe power consumption is 10 W.

The preferred embodiment may comprise a micro-electro-mechanical system(MEMS) mirror 20 coupled to a low-cost pulsed erbium fiber laser. Thepreferred embodiment may utilize, for example, a 5-6 Hz frame rate, animage size of 256 (h)×128 (v) pixels, a 42°×21° field of regard, 35 mrange, optional eye-safe operation, and 40 cm range resolution withprovisions for super-resolution. The preferred embodiment may be used inconjunction with ground robots and unmanned aerial vehicle (UAV)applications. The data acquisition system has the ability to capturerange data from three return pulses in a pixel (that is first, last, andlargest return), and information such as elapsed time, operatingparameters, and data from an inertial navigation system. Optionally, thepreferred embodiment may comprise additional performance subsystems toobtain eye-safety certification. To meet the enhanced range requirementfor the UAV application, receiver circuit of the preferred embodimentimproves the signal-to-noise (SNR) several-fold over the prior artdesigns. A low-capacitance large area detector may be utilized to enableeven further improvement in receiver SNR. The range capability iscontemplated and estimated to be 160 m. Optionally, the preferredembodiment ladar may be integrated with a color camera and inertialnavigation system to build a data collection package to determineimaging performance for a small UAV.

The laser output fiber may be connected to the input fiber of a gradientindex (GRIN) collimator to produce a light beam narrow enough to fitwithin the diameter of the MEMS mirror. Collimator 35A (as shown in FIG.6) may, for example,have a beam diameter 0.4-0.5 mm and the divergenceof 2 milliradians (mr). The collimator may be, for example, 3 mm indiameter and 13 mm long; which allows reduction in the size of thetransmitter assembly.

The collimated light is directed onto the face of the MEMS mirror. Themirror is 1.2 mm in diameter and can be scanned in two axes up to +/−6degrees or equivalently 12 degrees peak-to-peak. The overall MEMS chipsize may be 4.5 mm square. The scan rate may be approximately 700 linesper second which is sufficient to form 256×128 pixel images at a 5-6 Hzrate. The mirror is steered by drive voltages generated from a digitalrepresentation of the scan pattern stored in a memory. These digitalnumbers are mapped to analog voltages with a digital-to analog convertorand amplified with a high voltage operational amplifier.

To increase the amount of scan angle the reflected laser beam is passedfrom the mirror into a beam telescope. Using the designer's model forthe GRIN lens and an optical modeling code (Zemax), a lens combinationfor the beam telescope may be utilized that scans a beam +/−30° with+/−12° scan of an input beam. Small diameter lenses may be utilized toreduce the completed size of the transmitter subassembly. The firstelement in the beam telescope is a 12.7 mm diameter convex doublet witha focal length of 25 mm and the second is a 12.7 mm double concave lenswith a focal length of −9.7 mm. The output beam divergence isapproximately 4 mr which roughly matches the desired pixel format.

Light reflected from the image scene is detected with a receiveraperture that has a field-of-view encompassing the entire scanning rangeof the transmitter. This is a classic bi-static ladar architecture whichsignificantly eliminates numerous optical components and requires noprecision alignment as compared to the mono-static architecture whichshares a common transmit and receive aperture. The bi-staticarchitecture also allows the receiver aperture to be made larger thanthe MEMS mirror which is the limiting aperture size for the mono-staticarchitecture. Larger receiver apertures allow for higher signal-to-noiseand therefore increased LADAR range capability for a given laser power.

A fiber bundle may be used instead of the traditional lens approachbecause for large field of views (FOVs (45°-60°), a lens will yield onlysmall amounts of optical gain (1.4 for example for a 45° FOV) whereasthe fiber bundle with an input to output diameter ratio of threeprovides gains of 4-5. To further increase the amount of captured lightthe receiver uses a large area photodetector. Unfortunately largephotodetectors present large capacitive loads to the input circuit ofthe following amplifier that limit its bandwidth to less than what isrequired to pass a 2-3 ns pulse. The key to achieving the necessarybandwidth is a feedback circuit 92 (comprised of a series resistor,capacitor, and inductor) connected between the input and the output of alow noise monolithic microwave integrated circuit (MMIC). The capacitoris used to simply block the difference in the DC levels between theamplifier input and output. The inductor opens the feedback path at highfrequencies, where phase shift in the amplifier is sufficient to causeinstability, and the resistor acts as the feedback element at thefrequencies of interest. This receiver technique provides bettersignal-to-noise ratio (SNR) than the standard techniques using a smallphotodetector with a high gain transimpedance amplifier. Because theoutput diameter of the fiber taper is large, a two-fold improvement inSNR follows from placing four photodetectors behind the taper output andcombining the outputs of four detector/amplifiers. The receiver designuses four 1.5 mm diameter PIN InGaAs photodetectors and providessufficient SNR to detect average reflectivity targets to about 35-40 min range.

The output from the combiner is followed by a lowpass filter stage tofurther increase SNR (refer back to FIG. 1). The filter output isseparated into a low and a high gain channel where the purpose is toaccommodate the large range of return signal levels caused by acombination of the scene reflectivity and space loss between near andfar range targets. The output of each gain channel is a limitingdifferential amplifier to prevent large return signals from damaging thefollowing analog-to-digital convertor.

The differential amplifiers respectively feed two analog-to-digitalconvertors (ADC) on the ADC/FPGA board. The ADCs may for example be aNational Semiconductor chip (ADCO8D15000) which is dual channel andsamples at a rate of 1.5 giga-samples per second (GSPS) to 8-bitresolution. Because the laser pulse signals recovered by the receiverare unipolar, the differential output amplifiers on the receiver arebiased with an offset current to enable sampling of the signals to thefull 8-bit resolution available.

The output of the ADC is continuously clocked into the FPGA Double DataRate (DDR) input registers. The data acquisition starts when the FPGAfires the laser and waits a predetermined latency period to begin toclock the DDR output into the input first-in-first-out (FIFO) register,which is 500 samples deep. The 500 samples, taken 0.667 ns apart, allowtargets to be acquired up to 50 meters. After the FIFO is filled, it isthen read-out and the samples are tagged with the output of a 9-bitrange counter, which was started when the laser fired. The low gainchannel is multiplied by a factor to approximate the difference in RFgain between the two channels. The two channels are combined into asingle 14-bit wide channel for further processing. This combined channelis a correlated against a kernel with 5 coefficients that represent theshape of the transmitted laser pulse. This “matched filter” facilitatesthe recovery of signals in noise. The output of the correlator is thenpassed through a threshold circuit to detect the largest target returns.Given that multiple returns from a single transmitted laser pulse for apixel is possible, one of three pulse types is selected for furtherprocessing: the first return received after a laser pulse istransmitted, the last return, or the highest amplitude return. All threereturn types may be processed but use of one of three is permissible.The selected return is then fed to an interpolation circuit to increasethe LADAR range accuracy. The interpolator works by fitting the signalreturn to a parabola that has a similar shape and then finding the rangevalue corresponding to the maximum of the fitted curve. The result is arange word that is represented with 13 bits instead of 9.

The field programmable gate array (FPGA) also controls the movement ofthe MEMS mirror. The code to set the mirror position uses a mirror scanmemory map that contains the vertical and horizontal scan positions forthe mirror as well as embedded code needed by the digital-to-analogconvertor (DAC). The DAC converts the digital data to an analog signalneeded to drive the mirror. Also embedded are the image synchronizationsignals H-sync and V-sync needed by the FPGA processor. H-sync andV-sync are the familiar horizontal and vertical signals. For horizontalimage lines, the mirror is scanned in a serpentine pattern using acosine wave. This technique eliminates ringing in the mirror movementthat would be caused by the strong, high frequency transient in theflyback of a raster scan. The left and right edges of the scan areblanked where the mirror is moving very slowly at the turnaround.Display software linearizes the horizontal pixel data by compensatingfor the sinusoidal motion of the mirror and reverses every other line tocorrect for the serpentine motion. For the preferred embodiment verticalscan, a linear sawtooth wave may be utilized where during theslowly-changing portion of the sawtooth the vertical position of themirror is incremented by one line during each horizontal blankingperiod. A short flyback portion of the vertical scan is blanked. TheH-sync encoding denotes the first pixel in a line, while V-sync isencoded in the first pixel of each frame. To prevent miss-registrationof the image data, a frame count and a delimiter is added to the datafed from the FPGA to the Ethernet board. The range and amplitude wordsare then sent to an output FIFO, which buffers the data between the187.5-MHz input data rate with the 2 z input rate of the Ethernet board.

The MEMS-scanned LADAR enclosure dimensions may be approximately 7.1″wide by 6.5″ deep by 3.1″ high; and total unit power consumption isabout 20 W.

Recovery of Multiple Returns

The preferred embodiment LADAR has the capability to simultaneouslyprocess multiple target returns per pixel. Up to three target returns,if present, are characterized as the first return to arrive, the lastreturn to arrive, and the largest amplitude return to arrive for a givenpixel. This added capability will allow foliage penetration and increasethe ability to see wires and other small objects.

The LADAR can also support a complete inertial system that includes atriaxal gyroscope, a triaxal accelerometer, and a triaxal magnetometer.This capability will allow multiple image frames to be ‘stitched’together when the LADAR is mounted in a moving platform.

Optionally, a 512 word header can be added to the output data streamthat will hold various system parameters such as laser power, time,frame number etc. This will aid in archival purposes

Detailed Description of Enhanced Receiver

The systems depicted in FIGS. 1 and 2 are representative of those inU.S. Pat. No. 8,081,301 wherein a fiber optic taper is placed in thelight path before the detector (photodiode 40 in FIG. 1) to increase theamount of detected light by a factor roughly related to the diameter ofthe input face to the output face of the fiber taper. Other non-imagingoptical devices such as light concentrators are suitable for thispurpose that perform similarly. Depending on the required ladarfield-of-view, short focal length lenses are also suitable to increasethe amount of light impinging on the photodiode. Light emitted from theoutput of the preceding optical device is converted into a photo-currentby the photodiode.

To raise the signal-to-noise of the receiver to meet ladar rangerequirements for a given laser power, the receiver uses a large areaphotodiode to collect as much light as possible at the output of theleading optical device to yield a higher photocurrent. For the class ofladars where high range resolution is required, the bandwidth of thephoto-current can range up to several hundred MHz. Achieving thisbandwidth is difficult for large area photodiodes because they havelarge parallel capacitances that in concert with the amplifier inputimpedance, create a circuit that sharply attenuates the photocurrentsignal. For instance a 3 mm diameter InGaAs PIN photodiode has acapacitance of 200 pF. This detector when fed directly into an amplifierwith a 50 ohm input impedance forms a circuit that will have a bandwidthof only 16 MHz.

The photocurrent may be fed directly into the gate of transistor 85A.Two resistors R1, R2 in series from the 3V to ground and the middle goesto the gate of transistor 85A. These two resistors form a “biasnetwork,” i.e. a combination of resistors used for biasing transistor85A. The bias resistors can be used to set a voltage at the gate oftransistor 85A to control the bias current of the transistor. To extendthe receiver bandwidth to capture short light pulses (3 ns for example),a negative feedback or bypass circuit is utilized (similar to that inU.S. Pat. No. 8,081,301, herein incorporated by reference). The originaland the new photocurrent amplifiers are both negative feedback designs.In the circuit described in the '301 patent an MIMIC amplifier wasutilized with a 50 ohm input resistance. In the simplest sense a 160 ohmresistor is connected between the input and the output of the amplifier.For the operating frequencies of the application 160 ohms in thefeedback was possible. An analysis of the circuit reveals that theimpedance presented at the input of the amplifier by the feedbackresistor is roughly the resistance divided by the amplifier gain. Theamplifier gain is a bit more than 10 so the presented impedance is 16ohms or less. This means that the photocurrent mostly flows through thefeedback resistor, that is I_(amp)=V_(in)/50 versusI_(feedback)=V_(in)/16. For the new photocurrent design of the preferredembodiment shown in FIGS. 6A, 6B, the input resistance of the E-PHEMT isvery high with respect to 50 ohms and thus almost all of thephotocurrent is drawn through the feedback resistor. R3.

Because of the feedback connection, a small output voltage change isreturned back to the amplifier input through the resistor R3. A positivechange in the output in the circuit results in a positive change at theinput where the output signal is fed back. A change at the input willproduce a negative change in the output voltage. Because the initiallyassumed change produces opposite results when propagated through thefeedback loop, a signal that opposes and tends to cancel the originallyassumed change correlates to negative feedback.

Note that in place of the MMIC amplifier (used in the '301 Patent), anEnhancement mode Pseudomorphic High Electron Mobility Transistor(E-PHEMT) 85A may be used with the associated negative feedback orbypass circuit 92A comprised of R3, L2, and C3 connected between thetransistor gate and the source. The E-PHEMT transistors have more gainthan the MMIC amplifier which enables the use of a larger feedbackresistor (R3) for the same bandwidth. The values of the resistor R3 andinductor L2 operate to control or determine the amplifier's (85A)photocurrent gain and bandwidth. Preferably the gate impedance is highenough such that only a small proportion of the current from thesensor/photodiode 40A passes into the gate and through the amplifier 85Awhile a significantly larger portion passes through the bypass circuit92A. The decoupling capacitor (e.g. C3) is configured to allow the biasof amplifier/transistor 85A to be established by the at least onebiasing resistor, such as resistors R1 and R2. A large value forresistor R3 improves the noise performance of the photocurrent amplifierby reducing the Johnson noise from the resistor. E-PHEMTs also have ahigher input impedance (2-4 pF capacitor versus 50 ohms for the MMIC)that causes almost all of the photocurrent to flow through the bypass ornegative feedback circuit 92A. This contributes to raising the feasiblevalue of the feedback resistor R3 and thus reducing noise. Additionally,the noise figure of the E-PHEMT is also lower.

Significant increase in SNR is achievable by reducing the inductance, L5to the lowest value possible. This can be done with careful layout ofthe receiver circuit board. The best approach is achieved when thephotodiode is glued to a pad on the board and the anode is wire bondedto the Q1 gate of transistor 85A. Another method is to mount thephotodiode in a leadless chip carrier which is then soldered to the leadon the board running to the E-PHEMT gate. The amplifier (transistor 85A)bandwidth may be extended by with judicious choice of the inductance,L2; which may be determined using circuit modelling codes such as SPICE.

Additional SNR is achieved through another means which is unique to thisphotocurrent amplifier. A second transistor/amplifier 85B, which alsomay be for example an E-PHEMT, is used as a buffer between 85A andfollowing receiver circuitry thereby decoupling transistor/amplifier 85Afrom devices using standard 50 ohm input impedances. Resistors R4, R5can be used to set a voltage at the gate of transistor 85B such thatwhen the supply voltage is 3V (as shown in FIG. 4B) one can use tworesistors R4, R5 in series from the 3V to ground and the middle goes tothe gate of transistor 85B. The two resistors R4, R5 are called “biasresistors” since they “bias” the gate. These two resistors form a “biasnetwork’ i.e. a combination of resistors used for biasing a transistor.By biasing transistor/amplifier 85B with high value resistors, R4 andR5, a high load impedance is presented to transistor/amplifier 85A thateffectively allows the amplifier to have more voltage gain relative towhen it is loaded with 50 ohms. This increase in gain allows the use ofa larger feedback resistor (R3) for the same bandwidth thereby improvingthe SNR.

The preferred embodiment enhanced receiver circuit is amenable totechniques to improve the overall ladar receiver SNR by combining theoutputs of several detector/amplifier circuits. As another approach, theinputs of multiple amplifiers circuits can be connected to one largephotodiode. The outputs of transistors/amplifiers 85A, 85B can then besummed to raise the overall SNR of the ladar receiver. The presentinvention is a clear improvement in terms of SNR relative to U.S. Pat.No. 8,081,301 “Ladar Architecture” disclosure. With the prior design,the ladar could image to only about 30 m. Breadboards of the enhancedreceiver where the SNR is at least a factor of four greater than theprior art show that imaging in excess of 50 m is possible.

Imaging capability to 160 meters is attainable. FIG. 6 is anillustration of a tapered light concentrator. A light concentrator isused as a low-cost alternative to fiber taper. The optical gain wasfound to exceed the gain of the fiber taper by a factor of two.Additional gain is achieved by using an index matching fluid between theoutput of the concentrator and the detector. Tests of a preferredembodiment receiver including the concentrator and index matching fluidagainst a 0.6 reflectivity a target at 10 m with a 1 kW laser pulseyielded a SNR equal to 448. When this result is extrapolated to a 20%target using four combined receivers and a 5 kW laser pulse, the rangefor a SNR of 5 is 173 m.

Eye-Safety System

Referring now to FIG. 7, an eye-safety system was designed and installedin the three brassboard MEMS-scanned LADARS to eliminate the requirementfor the user to wear protective eye-wear. The purpose of the system wasto shut-down the laser output if the scan function failed to preventexposing anyone in the area to an ocular hazard arising from thenon-scanned laser beam. For the MEMS-scanned LADAR, the safety system isrequired to shut-down the laser within 39 ms after scanning ceased forthe present laser power.

A simplified block diagram of the preferred embodiment's optional eyesafety system is shown in FIG. 7. In the FIG. 7 embodiment, the sinewavehorizontal drive 135 and ramp vertical drive 136 for the MEMS mirror 20is fed into their respective high voltage amplifiers 137, 138 and twoADC channels 138, 139 in the microprocessor 134. Light from a low-powerred laser impinges on the MEMS mirror 20 and is reflected onto a siliconposition sensing detector (PSD) 131. Current output from the positiondetector is converted into a voltages proportional to the X and Ypositions of the red laser beam on the detector face and fed into ADCs141, 142 in a microprocessor 134.

The decision to shut-down the laser is performed by code on themicroprocessor. The ADCs sample the four input voltages at 4 kHz andload the data into four arrays that are 85 samples long. For thesinewave drive 135 and the PSD sinewave (132), the maximum and minimumof these two arrays are found over the first 20 samples of therespective data arrays. If the difference between the maximum andminimum is above 50 percent of the nominal value, the laser is permittedto continue operation. Both sinewaves are large signals with very littlenoise thus the standard deviation of the voltage differences are alsovery small and the probability of false alarm, that is shutting-down thelaser unwarrantedly is very low. For the drive ramp, the situationbecomes more complex because the change in signal voltage is small overthe measurement array. A technique that provides low false alarm dependson computing the ramp slope using a least squares fit over the firsthalf and second half of the data array. If the slope in both regions isbelow a set level, a command is sent to shut-down the laser. To preventfalse alarms when the sampled region includes the ramp flyback, theabsolute value of the slope calculated using the beginning and end ofthe half array is taken as the slope. The PSD ramp turned-out to be themost difficult signal because some of the sinewave signal hascross-coupled into it. In this case the slope using the least squaresfit is computed over the full length of the data array. This done fortwo successive data arrays and if the slope is below a set value forboth computations, a shut-down signal is sent. As with the drive ramp,the absolute value of the slope using the beginning and end values ofthe sampled file is computed to inhibit false alarms when the rampflyback is present.

In testing the eye-safety system, if both horizontal and vertical drivesignals are lost, laser shut-down occurred in about 20 ms. If the 150 Vsupply to the amplifiers 136, 137 driving the mirror scan failed,shut-down occurred also in about 20 ms. If the horizontal drive 135 tothe mirror failed, shut-down occurred in 20 ms and if the vertical drive136 failed, shut-down occurred in about 60 ms. This system was installedin the three brassboard LADARS.

FIG. 8 is an illustration showing an exemplary 4 hosel design for thelight concentrators 35A. Shown in FIG. 8 is a portion of a case for apreferred embodiment receiver that includes four hosels for holding thelight concentrators with a mechanism for allowing the introduction ofthe index matching gel (Shown). The scale (see ruler) in the figure is6″.

Large-Area and Low-Capacitance Detector

To further improve the SNR of the MEMS-scanned LADAR, shown in FIG. 9 isa large area low-capacitance detector or photodiode 40A used inconjunction with any of the preferred embodiments of the presentinvention. To the right in FIG. 9 are 5×5 arrays of 1×1 mm detectors.The 1×1 mm detectors may be connected with wirebonds. Regardingperformance characteristics, obtainable with the preferred embodiment,capacitance is less than <12 pF/mm2; responsivity >0.8 A/W; andbandwidth: >250 MHz.

Calculations show that such a design is possible where the capacitanceper unit area may be ⅕- 1/7th the capacitance of the InGaAs detectorsnow used. Thus for the same capacitance, detectors with 5-7 times thelight collection area are feasible that will provide the same bandwidthwhen coupled to the existing amplifier design. A rough calculation ofthe responsivity of these detectors (0.8 A/W) is slightly lower than forthe InGaAs detectors (0.95 A/W). This reduces the photocurrent from thelight collection area, but still leaves a significant SNR improvement of4.2-5.9 times the value for an equivalent capacitance InGaAs detector.This SNR increase maps to a 2-2.4 fold improvement in ladar range forthe same SNR at the maximum range.

A detector 80A using Mercury Cadmium Telluride (MCT) on Silicon isillustrated in FIG. 10. The architecture is based on the N+- on P doublelayer planar heterostructure (See FIG. 10, right). The technique toreduce detector capacitance is to have small N+-regions in the P-typeabsorber in a sparse arrangement as in FIG. 10, left. This significantlyreduces the junction capacitance relative to a standard photodiodedesign where the junction covers the entire detector area. Because themobility of the minority carriers is high, the diffusion length orcollection length, L is also high. L, however, places a limit on themaximum distance between the small junctions or lateral collectiondiodes to maintain good responsivity at the frequencies needed tocapture the laser light pulse. Fortunately, L is large enough that thedensity of the lateral collection diodes is low enough to achieve theestimates for detector capacitance previously mentioned. A laid-out amask for an array of 1 mm diameter detectors with the lateral collectiondiodes falling on a face-centered hexagonal close packed grid thatin-turn are tied together at the metal level.

A preferred embodiment of the present invention is directed toMEMS-scanned LADAR capable of performing ground surveillance from asmall UAV. For example, a preferred embodiment LADAR may have the size,weight and power (SWAP) of 7″×6.5″×3.1″; 1.4 lbs (exclusive ofenclosure) and 22 Watts which is suitable for unmanned aerial vehicleapplications. The rate at which the LADAR forms pixels is sufficient tocollect high fidelity imagery of the ground.

The LADAR package of the present invention may comprise a color cameraand inertial navigation system. Software will be written to fuse theladar data with the camera imagery and display the result in nearreal-time.

The preferred embodiment LADAR is capable of use in conjunction withsmall UAVs flying at low altitudes with powerful capabilities. Becausethe ladar is capable of collecting considerable redundant imageryfoliage penetration is possible. Since the UAV flies at altitudesbetween 100 to 200 meters above ground level (AGL), ground imageresolution of 30 cm is likely achievable. The system will be relativelylow-cost thus it can be owned and operated by small units. Thereal-time, high angular resolution and foliage penetration capabilitiesof the ladar/camera system will supply the user with high-qualityimagery for mission planning, real-time mission observation,organizational systems placement, threat or security activity andlocation, and surveillance of area/object detection and identification.

The ability of the preferred embodiment LADAR to capture the first,last, and largest return is likely to produce higher quality imagery andmore information when working in highly cluttered environments. The INSinformation simultaneously collected in close proximity to the LADARtransmitter allows a user to stitch together multiple overlapping orcontiguous images to improve penetration of foliage, eliminate shadows,and form registered large area images. Circuitry added to detect scanfailure and shut-down the laser was utilized to obtain Class 1certification from the US Army Public Health Command on the basis thatlaser emissions from the MEMS-scanned LADAR are not considered asignificant risk for injury during normal use. The preferred embodimentenhanced receiver design improves the SNR of the ladar receiver 4-5×that can be traded for reduced laser power for existing short-rangeapplications or more range for new applications such as for UAVs.Research to build a low-capacitance, large-area detector will havefurther impact on reducing laser power or extending range without theuse of high power lasers if the design proves feasible. Features of thepresent invention include multiple pulse detection, INS, eye-safety, andenhanced receiver provides support to recover quality imagery fromcomplex and cluttered scenes.

As used herein, MEMS means Micro-electromechanical systems, According toWikipedia:

-   -   Microelectromechanical systems (MEMS also written as        micro-electro-mechanical, MicroElectroMechanical or        microelectronic and microelectromechanical systems and the        related micromechatronics) is the technology of microscopic        devices, particularly those with moving parts. It merges at the        nano-scale into nanoelectromechanical systems (NEMS) and        nanotechnology. MFMS are also referred to as micromachines in        Japan, or micro systems technology (MST) in Europe. MEMS are        made up of components between 1 and 100 micrometers in size        (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size        from 20 micrometers to a millimeter (i.e. 0.02. to 1.0 mm). They        usually consist of a central unit that processes data (the        microprocessor) and several components that interact with the        surroundings such as microsensors. . . . At these size scales,        the standard constructs of classical physics are not always        useful. Because of the large surface area to volume ratio of        MEMS, surface effects such as electrostatics and wetting        dominate over volume effects such as inertia or thermal mass. .        . .

As used herein the term “target” means background, area of interest,zone of interest, location of motion, field of endeavor or the like.

As used herein, the terminology “photocurrent” means an electric currentinduced by the action of light; a stream of electrons produced byphotoelectric or photovoltaic effects.

As used herein, the terminology “circuit” means a path between two ormore points along which an electrical current can be carried.

As used herein, the term “subcircuit” means a distinct portion of anelectrical circuit; a circuit within another circuit.

As used herein, the terminology “negative feedback” means a process bywhich a portion of an outputted signal, which may be either a voltage ora current, is used as an input. Negative feedback is opposite in valueor phase (“anti-phase”) to the input signal.

As used herein, the terminology “high [point or element] impedance”means that a point in a circuit (a node) allows a relatively smallamount of current through, per unit of applied voltage at that point.

As used herein, the term “optimal” means most desirable or satisfactoryresult for an application or applications under specific conditions;resulting in the most favorable, reasonable conditions for operation ofthe system or device.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of the claims.

1. A laser receiver comprising: a sensor comprising a photosensitiveregion and outputting a photocurrent; a first amplifier configured toamplify the photocurrent operatively connected to the sensor; the firstamplifier comprising a first gate, a first source and a first drain, theoperative connection between the sensor and the first gate beingconfigured to minimize inductance; the first gate being operativelybiased by at least one first biasing resistor; a first subcircuitoperatively connected between the first drain and the first gate; thefirst subcircuit comprising, in series; a first resistor, a firstinductor and a decoupling capacitor; the first resistor effecting thephotocurrent gain of the amplifier; the first inductor effecting thebandwidth; and the decoupling capacitor being configured to allow thefirst amplifier bias to be established by the at least one first biasingresistor; the impedance of the first gate being high enough such thatonly a small proportion of the current from the sensor passes into thefirst gate and through the first amplifier while a significantly largerportion passes through the first subcircuit; an inductor connecting thegate to the at least one biasing resistor with high impedance at thereceiver operating frequency; a second amplifier comprising a secondgate with a gate impedance greater than 200 ohms operatively connectedto the first drain; at least one second biasing resistor connected tothe second gate with high impedance for the purpose of decreasing theload impedance on the first amplifier and thus increasing the voltagegain of the first amplifier; and thus enabling an increase in the valueof the first resistor that in-turn improves the signal-to-noise of theamplifier; the first and second amplifiers having a combined bandwidthsufficient to pass a modulated photocurrent at a frequency up to 500MHz; the second amplifier comprising an output configured to beoperatively connected to a processing unit and a display unit displayingoutput from the processing unit in the form of pixels, the display beingof sufficient quality to display an image of a target at 1000 meters;whereby the receiver is configured to receive light reflected from atarget illuminated by a laser configured to emit light at an intensitysafe for human eyes and wherein a target is discernible in a range from0.3 to 1000 meters.
 2. The receiver of claim 1 wherein the receivercomprises at least one of a tapered fiber bundle or light concentratoroperative to receive light operatively connected to sensor; and whereinthe receiver is configured to receive light of a predetermined frequencyand modulation format reflected from a target illuminated by a laser;and wherein the first subcircuit is a negative feedback circuit.
 3. Thereceiver of claim 1 wherein the sensor comprises a large areaphotosensitive region with a diameter ranging from 1 mm to 10 mm andoutputs a photocurrent and wherein the first resistor and the firstinductor operatively effect the photocurrent gain and bandwidth of thefirst amplifier.
 4. The receiver of claim 1 wherein the capacitance ofthe photodetector is between 10 pF and 400 pF and wherein the firstamplifier is an enhancement mode psuedomorphic high electron mobilitytransistor.
 5. The receiver of claim 1 wherein the inductance betweenthe sensor and the gate is minimized by making the distance between thephotodetector and first gate approximately less than 1 cm.
 6. Thereceiver of claim 1 wherein the inductance between the sensor and thegate is in the range of 0.1 nH to
 10. nH.
 7. The receiver of claim 1wherein the first resistor is in the range of 20 to 2000 ohms and thefirst inductor is in the range of 50 nH to 2000 nH such that theimpedance of the bypass circuit causes the signal to noise ratio,amplifier gain, and bandwidth to be optimal for the receiverapplication.
 8. The receiver of claim 1 wherein the receiver is used tooperate a robot and wherein the receiver operates on battery powerproviding current in the range of 5 mA to 2000 mA in order to minimizethe power consumption on the robot.
 9. The receiver of claim 1 whereinthe receiver is sized so as to fit in an area of less than or equal to 3cm by 3 cm by 0.5 cm.
 10. The receiver of claim 1 wherein the receivercomprises a plurality of subreceivers and wherein the output of each ofthe plurality of subreceivers is substantially redundant and each outputis connected to a combining circuit that is operatively connected to theprocessing unit.
 11. The receiver of claim 1 operatively connected in acircuit to at least one other receiver, the at least one other receivercomprising a second light concentrator; a second sensor operativelyconnected to the second light concentrator, wherein the second sensorcomprises a photosensitive region and outputs a photocurrent; a thirdamplifier configured to amplify the photocurrent; the third amplifiercomprising a third gate, third source and third drain, the connectionbetween the sensor and the third gate configured to minimize inductance;the third gate being operatively biased by at least one third biasingresistor so as to create a high third gate impedance; a second subcircuit operatively connected between the third drain and third gate;the second subcircuit comprising in series: a third resistor, a thirdinductor and a third decoupling capacitor; the third resistor, thirdinductor, and third gate impedance being high enough such that only asmall proportion of the current from the sensor passes into the gate andthrough the third amplifier while a significantly larger portion passesthrough the second bypass circuit; the third decoupling capacitor beingconfigured to allow the first amplifier bias to be established by the atleast one first biasing resistor; an inductor connecting the gate to theat least one biasing resistor with high impedance at the receiveroperating frequency; a fourth amplifier comprising a fourth gate with ahigh gate impedance operatively connected to the by-pass circuit; atleast one fourth biasing resistor connected to the fourth gate with highimpedance for the purpose of unloading the third amplifier and thusincreasing the voltage gain of the third amplifier and thesignal-to-noise ratio; the third and fourth amplifiers having a combinedbandwidth sufficient to pass the modulated photocurrent; the fourthamplifier comprising an output configured to be operatively connected inparallel with the output of the second amplifier to the processing unitand the display unit displaying output from the processing unit in theform of pixels, the display being of sufficient quality to display animage of a target at 1000 meters; whereby each of the plurality ofreceivers operates in parallel with the at least one other receiver andwherein the output of each of the plurality of receivers issubstantially redundant and each output is connected to a combiningcircuit that is operatively connected to the processing unit.
 12. Thereceiver of claim 1, wherein the receiver has the capability tosimultaneously process multiple target returns per pixel.
 13. Thereceiver of claim 11 wherein each pixel of the display represents apoint in the area surrounding the target and wherein the light from thelaser reflected from the target area comprises the first return toarrive at the receiver for a given pixel, the last return to arrive fora given pixel, and the largest amplitude return to arrive for a givenpixel; and wherein the processing unit operates to select one or more ofthe returns which allows foliage penetration and increases the abilityto see wires and other small objects.
 14. The receiver of claim 1wherein the receiver is operated in tandem with an inertial system thatcomprises one of a triaxal gyroscope, a triaxal accelerometer, and atriaxal magnetometer.
 15. The receiver of claim 1 wherein the processingunit operates to join together multiple image frames when the receiveris mounted in a moving platform.
 16. The receiver of claim 1 configuredfor target applications such as three dimensional scene recording andsurveillance, crime scenes, architectural structures, agriculturalsubjects, troop placement, and autonomous machines, self-drivingautomobiles, manufacturing robots, battery powered unmanned aerialvehicles or robots.
 17. The receiver of claim 1 wherein the receiver hasthe capability to simultaneously process multiple target returns perpixel characterized as the first return to arrive, the last return toarrive, and the largest amplitude return to arrive for a given pixel.18. The receiver of claim further comprising a microwave combinercombining the photocurrent from each of the plurality of microwaveamplifiers to a combined output.
 19. A laser receiver comprising: asensor comprising a photosensitive region and outputting a photocurrent;a first amplifier configured to amplify the photocurrent operativelyconnected to the sensor; the first amplifier comprising a first gate, afirst source and a first drain, the operative connection between thesensor and the first gate being configured to minimize inductance; thefirst gate being operatively biased by at least one first biasingresistor; a first subcircuit operatively connected between the firstdrain and the first gate; the first subcircuit comprising, in series; afirst resistor, a first inductor and a decoupling capacitor; the firstresistor effecting the photocurrent gain of the amplifier; the firstinductor effecting the bandwidth; and the decoupling capacitor beingconfigured to allow the first amplifier bias to be established by the atleast one first biasing resistor; the impedance of the first gate beinghigh enough such that only a small proportion of the current from thesensor passes into the first gate and through the first amplifier whilea significantly larger portion passes through the first subcircuit; aninductor connecting the gate to the at least one biasing resistor withhigh impedance at the receiver operating frequency; a second amplifiercomprising a second gate with a gate impedance greater than 200 ohmsoperatively connected to the first drain; at least one second biasingresistor connected to the second gate with high impedance for thepurpose of decreasing the load impedance on the first amplifier and thusincreasing the voltage gain of the first amplifier; and thus enabling anincrease in the value of the first resistor that in-turn improves thesignal-to-noise of the amplifier; the first and second amplifiers havinga combined bandwidth sufficient to pass a modulated photocurrent at afrequency up to 500 MHz; the second amplifier comprising an outputconfigured to be operatively connected to a processing unit and adisplay unit displaying output from the processing unit in the form ofpixels, the display being of sufficient quality to display an image of atarget at 1000 meters; whereby the receiver is configured to receivelight reflected from a target illuminated by a laser configured to emitlight at an intensity safe for human eyes and wherein a target isdiscernible in a range from 0.3 to 1000 meters.
 20. A method foroperating a laser receiver comprising: a sensor, a first amplifieroperatively connected to the sensor; the first amplifier comprising afirst gate, a first source and a first drain, the operative connectionbetween the sensor and the first gate being configured to minimizeinductance; the first gate being operatively biased by at least onefirst biasing resistor; a first subcircuit operatively connected betweenthe first drain and the first gate; the first subcircuit comprising, inseries; a first resistor, a first inductor and a decoupling capacitor;the first resistor effecting the photocurrent gain of the amplifier; thefirst inductor effecting the bandwidth; and the decoupling capacitorbeing configured to allow the first amplifier bias to be established bythe at least one first biasing resistor; the impedance of the first gatebeing high enough such that only a small proportion of the current fromthe sensor passes into the first gate and through the first amplifierwhile a significantly larger portion passes through the firstsubcircuit; an inductor connecting the gate to the at least one biasingresistor with high impedance at the receiver operating frequency; asecond amplifier comprising a second gate with a gate impedance greaterthan 200 ohms operatively connected to the first drain; at least onesecond biasing resistor connected to the second gate with high impedancefor the purpose of decreasing the load impedance on the first amplifierand thus increasing the voltage gain of the first amplifier; and thusenabling an increase in the value of the first resistor that in-turnimproves the signal-to-noise of the amplifier; the first and secondamplifiers having a combined bandwidth sufficient to pass a modulatedphotocurrent at a frequency up to 500 MHz; the second amplifiercomprising an output configured to be operatively connected to aprocessing unit and a display unit displaying output from the processingunit in the form of pixels, the display being of sufficient quality todisplay an image of a target at 1000 meters; the method comprisingreceiving light reflected from a target in a range from 0.3 to 1000meters; processing and displaying the output.